1. Field of the Invention
The present invention relates to a pulsed light source using a laser diode for generating a short pulsed light (200 to 2 picosecond pulse width, for example) of a high repetitive frequency, and more specifically to a low noise pulsed light source capable of generating an optical pulse with reduced light intensity noise.
2. Description of the Prior Art
An emitted light from a laser diode (LD) changes in its wavelength and intensity as an excitation current and ambient temperature vary. The intensity of the emitted light also changes owing to the competition among longitudinal modes and owing to mode hopping. As a method for reducing such variations of the light intensity, there is known a technique wherein a photodetector element such as a photodiode (PD) detects part of the emitted light from a laser diode to estimate an error signal between a detected light intensity level and a preset one which error signal is in turn fed back to an excitation current source which is to drive the laser diode. Such a technique has already been used for an optical pick-up of a compact disk (CD) player and so on.
However, all prior practice to reduce the variations of the light intensity was applied to a laser diode for emitting continuous wave (CW) light or direct current (DC) light. Up to now, no investigation was made of the noise in the intensity of such pulsed light, and no trial was made to stabilize the pulsed light intensity.
On the other hand, there are many application fields in need of short pulsed light because of temporal resolution being specified by the width of pulsed light. Those fields include an electro optic sampling technique as disclosed in IEEE Journal of Quantum Electronics, Vol. QE-22, No. 1, January 1986, PP 69 through 78 in which an ultrashort light pulse is used as a sampling gate to nondestructively measure an electric signal with use of an electro-optic (E - O) effect; a fluorescence life measuring technique as disclosed in Rev. Sci. Instrum. 59 (4), Apr. 1988, PP 663 through 665 in which an ultrashort light pulse is used to measure laser excited fluorescence; estimation of response characteristics of photoelectric detectors and optical intergrated circuits (OE IC), etc.; time correlated photon counting method using a photomultiplier, and so on, for example. A dye laser which generates a picosecond to femtosecond width pulsed light is usable for such applications from the viewpoint of time resolution but with a difficulty of its being large-sized. Instead of this, laser diodes are hopeful as pulsed light sources, because they have some advantages of their being simple and small-sized in structure, inexpensive in manufacture.
Now, laser diodes can generate a short pulsed light with an about 200 to 20 picosecond width, and with about 670 nm to 1.5 .mu.m wavelengths being typical, the latter emission wavelengths being varied depending upon the kinds thereof. Additionally, a second harmonic of the pulsed light from a laser diode is available to assure a short wavelength pulsed light up to 340 nm. Repetitive frequencies of such light pulses generally range from 0.1 to 200 MHz although being different in accordance with applications. Furthermore, there are technically available GHz high repetition pulsed light.
The present inventors have however experimentally found that use of such a high repetition optical pulse causes measured intensity fluctuations of the light pulse so as to limit the accuracy of a pulse light intensity measurement described below particularly with respect to FIG. 12. For simplicity, there will be described a measurement of transmittance of a pulsed light through a sample 10 with use of a device illustrated in FIG. 10. In FIG. 10, a laser diode 12A (refer to FIG. 11) incorporated in a laser diode (LD) pulsed light source 12 emits the optical pulse which is controlled in its repetitive frequency by an oscillator 14 (repetitive frequency 100 MHz, pulse width 50 picosecond, and wavelength 830 nm, for example). The LD pulsed light source 12 is constructed as illustrated in FIG. 11, for example, to which a bias current has previously been supplied and on which a negative pulse is applied from an electric pulse generator 12B (Hewlett Packard, 33002A Comb-Generator (registered trademark) for example) using a step recovery diode for example to drive the LD 12A.
The pulsed light emitted from the laser diode (LD) 12A impinges upon the sample 10 through a chopper 16 (chopping frequency 1 kHz, for example) driven by the oscillator 15 and is partly absorbed by and partly transmitted through the same as an output light. The output light is focused by a lens 18 and detected by a photodetector 20 composed of a photodiode (PD) for example. An output signal from the photodetector 20 is amplified by a low noise amplifier 22 and lock-in detected by a lock-in amplifier 24. A chopper signal generated by the oscillator 15 is used for a reference signal in the lock-in amplifier 24. Herein, photoelectric current noises produced in the photodetector 20 and noises produced in the low noise amplifier 22 have been reduced by limiting noise passing through lock-in amplifier 24 to noise having a frequency within a predetermined frequency range defined by the bandpass of lock-in amplifier 24.
An output from the lock-in amplifier 24 is fed to an output meter 26 for example and displayed with respect to the transmittance of the foregoing output light.
Herein, although the foregoing device of FIG. 10 was made of the chopper 16 and of the lock-in amplifier 24 for lock-in detection for the purpose of the reduction of measurement noises and the improvement of measurement accuracy, such construction is unnecessary in simple measurements. In other words, an output from the photodetector 20 may be amplified and read in a direct manner. Further, the low noise amplifier 22 may be omitted and the lock-in amplifier 24 may instead be employed.
In such a device, in a case where the transmittance of the pulsed light through the sample 10 is nonlinear with respect to the incident pulsed light intensity, and when it is required that the incident pulsed light intensity be accurately measured, it is required that pulse light intensity be measured with a sufficiently low level of the intensity fluctuation so that a desired measurement accuracy is achieved. Thereupon, a difficulty was discovered in the measurement process, the difficulty being that photoelectric current noise involved in the pulsed light emitted from a pulsed oscillation LD limits the sensitivity of the measurement.
Referring to FIG. 12, exemplary noise characteristics of the LD pulsed light obtained experimentally by the present inventors are illustrated, with the horizontal axis being frequencies and the vertical axis being effective values (rms) of photoelectric current noise in decibel (dB). The point O dB on the vertical axis indicates a shot noise level defined by the square root of the number of photons involved in the optical pulse (theoretical limit). FIG. 12 therefore indicates a noise level of the LD pulsed light normalized by the shot noise level. FIG. 12 further illustrates a noise level with the prior system as indicated by a solid line A and marks X. It is understood from the figure that the noise level when the LD undergoes pulsed oscillation is at least 10 times greater (20 dB) than the shot noise level, so that it is desired that the former be reduced to the latter shot noise level.
The data illustrated in FIG. 12 is given by measuring photoelectric current noise produced when the LD 12A is driven by a driving circuit 30 constructed as illustrated in FIG. 11 using a noise fraction measuring device composed of the photodetector 20, low noise amplifier 22, lock-in amplifier 24, an oscillator (OSC) 32 for frequency sweep, a noise detection circuit 34, and a display 36 as shown in FIG. 13.